1. Field of the Invention
The present invention relates to the optical system in a laser scanning eye fundus camera which scans the fundus of a subject's eye with an illuminating laser beam.
2. Description of the Related Art
Conventional eye fundus cameras are arranged to uniformly illuminate the whole of the subject's eye fundus so as to observe or photograph it. The eye fundus must then be illuminated with a relatively intense flux of light which might regrettably cause pain to the subject.
In recent years, a new type of eye fundus camera has been developed which reduces such pain. It is adapted to scan the subject's eye fundus with an illuminating laser beam having a constant spot diameter and to detect the reflected light from the eye fundus at constant intervals, each interval being equal to the spot diameter of the laser beam. The data derived from the reflection due to spot illumination is used to construct an image of the fundus of the subject's eye on a monitor cathode ray tube (CRT).
Meanwhile, as illustrated in FIG. 4, the depth of focus for such a laser beam becomes shallower (space A) as the spot diameter, or the diameter of the cross section, of the laser light beam and hence the F-number are reduced (indicated by D.sub.1). It conversely becomes deeper (space B) as the spot diameter and hence the F-number are increased (indicated by D.sub.2).
The eye fundus is not smooth but rugged. To obtain a clear image of the eye fundus, therefore, the depth of focus for the laser beam should be deeper and hence the spot diameter of the laser beam should be larger to some extent.
In prior art apparatuses of the last-mentioned type, however, the laser beam diameter is invariably set to a very small value in order to obtain signals due to spot illumination greater in number than the number of picture elements of the associated monitor CRT. As a result, there is a problem with the prior art apparatus in that the depth of focus is relatively shallow and the image of the subject's eye fundus can become out of focus if the eye slightly moves relative to the apparatus. There is a tendency for young persons, in particular, to move the head or eyes while undergoing the ophthalmological inspection or test. Thus, it is desirable for the apparatus to have means for varying the spot diameter such that a clear image of the eye fundus can be obtained in spite of possible slight movement of the subject's eye relative to the apparatus. It is also desirable for the apparatus to be able to reduce the spot diameter so as to achieve a high resolution.
An eye fundus camera of such type is known in which, for purpose of achieving a high contrast image, a light beam scanning device is used by both an illuminating optical system for emitting a laser light beam, and a light receiving optical system for directing the reflected light or fluorescence from the eye fundus. In this eye fundus camera, a pin hole or diaphragm is provided in the light receiving optical system at a position which is optically conjugate with the eye fundus under test. The diameter of the pin hole determines the diameter of the spot of light projected onto the eye fundus, or the size of each picture element, whereby the resolution is determined.
In other words, the light receiving optical system of the eye fundus camera of the above-described type is designed such that only the light reflected from a particular region of the eye fundus, i.e., the region onto which an image of the pin hole is projected by the light receiving optical system, can pass through the pin hole. Thus, even if the illuminating light spot on the eye fundus is greater than the pin hole image on the eye fundus, the light reflected from outside the region of the pin hole image cannot pass through the pin hole. The resolution of the image of the eye fundus is therefore determined by the pin hole image on the eye fundus. Thus, the resolution of the apparatus of such an arrangement is determined by the optical system having the pin hole, i.e., the light receiving optical system.
In order that the eye fundus camera of such type can receive as large an amount of reflected light as possible at its light receiving part, and thereby have a high resolution, the diameter of the laser beam from the illuminating optical system should be variable in accordance with the scanning magnification for the subject's eye fundus, and further the diameter of the aperture of the light receiving optical system should be as large as possible.
There is however a problem in that, as the aperture diameter of the light receiving optical system is increased, the resolution becomes higher but at the same time the depth of field becomes shallower, with the result that the whole of the eye fundus cannot be in focus at one time and it will be out of focus due to only slight movement of the subject's eye relative to the apparatus.